perf(prompts): make /tags fetch only the tags column with SQL access filter (#24287)

Non-admin GET /api/v1/prompts/tags went through get_prompts_by_user_id,
which loaded every active prompt with its full content/data/meta plus
owner records and all access grants, then ran one has_access query per
prompt that wasn't owned by the caller - all so the endpoint could
collapse the result to a sorted tag list. With 600 prompts this took
several seconds while the admin path (a single SELECT) returned in <1s.

Add Prompts.get_tags_by_user_id which selects only the tags column and
applies the same EXISTS-based access filter used by /list. Also tighten
the admin get_tags to project just the tags column instead of full rows.
The endpoint is now one DB query (plus one for groups), no row hydration,
no N+1.

Co-authored-by: Claude <noreply@anthropic.com>

backend/open_webui/routers/prompts.py

@@ -61,13 +61,7 @@ async def get_prompts(user=Depends(get_verified_user), db: AsyncSession = Depend
 async def get_prompt_tags(user=Depends(get_verified_user), db: AsyncSession = Depends(get_async_session)):
     if user.role == 'admin' and BYPASS_ADMIN_ACCESS_CONTROL:
         return await Prompts.get_tags(db=db)
-    else:
-        prompts = await Prompts.get_prompts_by_user_id(user.id, 'read', db=db)
-        tags = set()
-        for prompt in prompts:
-            if prompt.tags:
-                tags.update(prompt.tags)
-        return sorted(list(tags))
+    return await Prompts.get_tags_by_user_id(user.id, db=db)
 
 
 @router.get('/list', response_model=PromptAccessListResponse)